High density integrated circuitry is principally fabricated from semiconductor wafers. Upon fabrication completion, a wafer contains a plurality of identical discrete die areas which are ultimately cut from the wafer to form individual chips. Die areas or cut dies are tested for operability, with good dies being assembled into separate encapsulating packages which are used in end-products or systems.
One type of integrated circuitry comprises memory. The basic unit of semiconductor memory is the memory cell. Capable of storing a single bit information, the memory cell has steadily shrunk in size to enable more and more cells per area of a semiconductor substrate or wafer. Such enables integrated memory circuitry to be more compact, as well as faster in operation.
Example semiconductor memories include ROMs, RAMs, PROMs, EPROMs and EEPROMs. Some emphasize compactness and economy over speed. Others focus on lightening-fast operation. Some store data indefinitely, while others are so temporary they must be refreshed hundreds of times every second. The smallest memory cell comprises the single transistor capacitor of a dynamic random access memory (DRAM).